Telephone transmitter and receiver



May 2 946- J. s. P. ROBERTON ET AL. 2,400,662

TELEPHONE TRAIISEJITTER AND RECEIVER Filed May 18, 1942 Patented May 21,1946 UNITED: STATES PATENT oFFics 2,400,662 TELEruoNE TRANSMITTER ANDRECEIVER James Samuel Paterson Roberton4 and Michael Lawrence Gayford,London W. C. 2, England. assignors to International StandardllectricCorporation, New York, N. Y.

Application May 1s, 1942, serial No. 443,424 in Gmt Britain May 27, 19415 claims; `(ci. 17e-115.5)

This invention relates to electro-acoustic transducers, that is to sayto telephone transmitters and/or receivers, of the moving-coil type; andconsists in improvements in the manu vfacture of such transducerswhereby the poleterial, a permanent magnet A disposed concen' tricallywithin the pot so that `the rim of the..

type with the required degree of accuracy inv the spacing of the innerand outer vpole-pieces, and

tape, which is iixed upon the rear or underside of a'diaphragm I6.

The central part of the diaphragm I6 is domed forwardly; and the spacethus formed between the rearslde of the diaphragm and the upper surfaceof the inner pole-piece I3 is partly filled by a segment I1, preferablyof synthetic-resin moulded material, which is held in place by thecentralvnut and bolt I4, with its flat surface against the flat top ofthe inner pole-piece. v.For

acoustic reasons the segment has within it a hollow space communicatingthrough slits with the space behind thel diaphragm. The hollow space maybe provided by a recess formed in the otherwise flat underside of themoulded segment, and the slits by the introduction of a spacing shim atthe position I8 surrounding the Vbolt I4 or by slight projections spacedaround the underside of the periphery of the segment.

' the pot into the space around the magnet II.

more especially with such mechanical rigidity In order that theinvention may be clearly understood, a description will now be given oftwo I of its embodiments as shown inlthe .accompanying drawing, inwhich:

Fig. l shows the rst embodiment in transverse section;

Fig. 2 the second embodiment in rear end view with the cover removed;

Fig. 3 a transverse section on the Fig. 2; and l Fig- 4 a part-sectionon the line 4-4 of Fig. 2.

As shown in Fig. 1, a short cylindrical magnet I I of high coerciveforce material carries an outer pole-piece in the form of a pot I2iitted over the magnet from its lower end and an inner polepiece in theform of a anged disc I3 iitted onto the magnet at its upper end. Themagnet and line 3-3 of The three parts of the magnet system are thusfirmly embedded in the hard resinous material, and this has the valuableeffect of fixing the parts rigidly with respect to one another and thusassuming the maintenance of a uniform air-gap, it being noted that, with.the powerful permanent magnets which modern materials make possible,any initial lack of concentricity of the pole-pieces at the air-gap isapt to become rapidly aggravated because the magnetic attraction acrossthe air-gap is at its -greatest on that radius at which the gap isnarrowest. This design also permits the acoustic properties to Ibeimproved: the moulding material is allowed to Ilow into the pot up to apredetermined contour whereby the proper volume of air space inside thepot (behind the moving coil) is secured. The holes in thev and outsideof the unit a small channel or holey may be provided in the front coveror mouthpiece to allow air to pass in or out from the chamitsDole-pieces are clamped' together by a central 55 bers between themembrane and the front of the nut and 'bolt I4. The inner and outerpolepieces I3 and I2, which are of high permeability material, deiinebetween them an annular airgap; and in this air-gap is located a speechcoil diaphragm. Pressure changes in the air behind the diaphragm will beprevented by air ow through the silk covered holes in the frame.

' This embodiment of the invention has been I5, in ther form of anedge-wound aluminium designed so as to be suitable for use on aircraft;

consequently one of the requirements is that lts magnetic field shallnot influence a compass even at short range. and the efficiency of themagnetic screening' is one ofA the features of this design.

In the first place it has been found that the pot-with-internal-magnetdesign (parts I2 and .I I) is very satisfactory in reducing the externalfield to a small value. However, there is inevitably some flux leakage,especially around the air-gap and to a lesser extent from the back ofthe pot pole-piece, and to reduce this still further if not eliminate itcompletely the magnetic structure is completely enclosed bymagnetic'screens. There is a front screen 22, which overlies thediaphragm I6, a back screen 23, which is in the form of a cup fittingover the insulating casing I9 and secured by screws, and a clamping ring24 which overlaps both of the other two and which, being spun intoposition, serves mechanically to hold the assembly together. It has beenfound necessary to have the screening surface as nearly possiblecontinuous all round the unit. The front screen 22 is of course in theform of a grid, i. e. it is apertured; but the holes in the mouthpieceor earpiece do not register with the holes in the front screen 22 sothat it is not possible to injure the diaphragm by poking anythingthrough the holes,

The leads from the speech-coil I are necessarily weak and fragile, Inthis design they are as short and as little exposed as possible, beingtaken to the tips of terminal screws 25 rigidly moulded in the frame;the winding end of the terminal is slightly below the level of theseating surface for the diaphragm. The back screen 23 forms a cover toprotect the terminals.

Figs. 2 to 4 show a Second embodiment of the invention. This is acapsule-type microphone, with one terminal of the speech-coil connectedto the back screen 23 and the other connected to an insulated centralcontact 26 in the back of said screen.

One of the interesting features of this design which does not appear inthat shown in Fig. 1 is the presence of the disc 21 of high permeabilitymaterial in the bottom of the pot I2. The purpose of this is to reducethe flux leakage which, as already mentioned, occurs from the back(base) of the pot. The flux passing from the central magnet to the rimof the pot is concentrated in the material of the pot, and thus issubjected to something of a bottle-neck at that part of the base of thepot which lies immediately below the periphery of the magnet. Furtherout the conditions become easier, although the thickness ofl metal isnot increased, simply because the annular cross-sectional area increaseswith the radius. The disc 21 serves to open up this bottle neck, thusreducing the concentration of flux, and also reducing the leakage. Itthen becomes possible to make the pot I2 of thinner material than wouldotherwise be necessary to carry a certain flux as prescribed by thethickness of the inner-'pole-piece I3. A thinner pot is easier ofmanufacture by a drawing operation, is of less Weight, and gives morespace behind for the terminals.

In this embodiment of the invention the diaphragm I6 seats upon a flangeout-turned from the rim of the pot instead of upon a moulded part. Also,the acoustic resistance is provided by the lambswool lling 28 of slotsin the moulding 29 which is secured to the pressed cup I2. This mouldingcarries the speech coil connections 30. In this construction there isonly the internal filling of setting material and no external casingeither continues with it or separate. The filling extends up on theoutside of the flange on the disc I3, instead of inside it as in thepreviously described construction.

The capsule is normally fitted in a, moulded holder which has a shortrubber horn. The design of this horn and the front holes and cavity ofthis holder has been considered inasfar as it affects the performance.The action of these in conjunction with the holes, cavities andprotective oiled silk (or substitute) membrane 2| al1 combine to enablethe desired response to be obtained, there being, in effect, a series offront resonators. The protective membrane 2I is supported at its centreby the perforated metal dome 20. As it is heid clear of this dome at theedges there is no trouble encountered through the membrane adhering tothe dome when saturated with moisture and failing to transmit sound.

The internal lling 3| of the cup is preferably formed in position byinjecting a suitable compound, such as plasticised polystyrene throughsmall holes in the cup, a ring being held in the magnetic gap duringthis process. By the use of suitable pressures and temperatures, the cupcan be filled without an undesirable flash being forced up the sides ofthe ring in the magnetic gap.

It will be understood that the invention may take other forms. Inrespect of the constructions already described for instance, the'movingcoil I5 might be of copper, and the dome I1 might be omitted.

What is claimed is:

1. An electro-acoustic transducer comprising a pot of magnetizablematerial, al permanent magnet disposed concentrically within the pot sothat the rim of the pot constitutes an outer pole, a disc ofmagnetizable material overlying the magnet in the mouth of the pot sothat its rim constitutes an inner pole, a diaphragm, a speech coilattached to said diaphragm and located within the annular air-gap formedbetween said poles, and

a filling in said pot of setting material rigidly.

holding said poles in concentricity. Y

2. An electro-acoustic transducer according to claim 1, wherein saiddisc is formed with a downwardly projecting flange and said fillingextends into the annular space between said flange and said magnet.

3. An electro-acoustic transducer according to claim 1, whereinapertures are provided in said pot which is embedded in a block ofsetting material which is continuous with said filling.

4. An electro-acoustic transducer according to claim 1, wherein said potis embedded in a block of setting material .and an aperture is bored insaid block to provide an air-passage through it extending through saidfilling into said air-gap.

5. An (ectro-acoustic transducer according to claim 1, wherein said potis embedded in a block of setting material and an aperture is bored insaid block from its outside surface to provide an air-passage through itextending through said filling into said air-gap, and wherein saidairpassage is covered with a layer of /silk applied over the outer endof the bored passage.

JAMES SAMUEL PATERSON ROBERTON.

MICHAEL LAWRENCE GAYFORD.

